LU-12904 build: Support for gcc -Wimplicit-fallthrough

LU-12904 build: Support for gcc -Wimplicit-fallthrough

Linux 5.3 enables -Wimplicit-fallthrough
Add decorators for implicit-fallthrough compiler checks.

Test-Parameters: trivial
Cray-bug-id: LUS-8042
Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Change-Id: I740062e60e1d19b967ec6b91970cdd3ab03cbab6
Reviewed-on: https://review.whamcloud.com/36577
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>